The ships built recently have been enlarged excessively in form for both large and small ships and their directional or course stability have tended to deteriorate.
The degree of directional stability of a ship has usually been determined by steering the ship according to the inverse spiral (or spiral) test or the like and utilizing data obtained from the relation between the rudder angles and the turning angular speeds of the ship.
Experience with the ships which were full-bodied beyond a certain extent has proved the occurrence of cases where the movement caused by steering will not be determined linearly and these cases are called as "unusual phenomena" in the learned circles, etc. These abnormally unstable ships (the ships which exhibit such unusual phenomena) have two or more stable steady turning conditions so that the movement of the ship caused by steering does not result in any single movement and a phenomenon of unstable ship motion occurs. According to the recent publications made in the learned circles, etc., this phenomenon is attributable to the fact that the ship has two or more stable turning resistances with respect to the same turning condition and it has also been said that the phenomenon is due to particular fluid forces which will be caused in the vicinity of the stern. An unusual phenomenon occurs when the relation between the movement of the ship's hull and the surrounding water current attains a certain condition so that the ship shifts to other stable points, and consequently the unusual phenomenon occurs intermittently at any time thus differing in phenomenon from the ordinary directional instability.
Considering the control of such ships having high degrees of directional instability from the standpoint of course keeping control, it is universally admitted that in the case of the ordinary directional instability, even if there exists a hysteresis loop for the steering characteristic of the ship, it is possible to stably control the course by inserting a sufficient phase compensating element in the control loop of the automatic steering system. However, in the case of ships which are subject to directional instability due to the unusual phenomena, it is impossible to stably control the course with a measure employing such a linear compensating element.
It has been announced that directional instability due to unusual phenomena is caused particularly by separation from the bottom surface or rounding of the water current accelerated by the propeller which is flowing to the vicinity of the stern of the full-bodied ship. In particular, considerable research works have been published with respect to the mechanism of forces which are produced laterally to the ships hull and effect on course keeping control.
It can be pointed out in common to these works that in the range where the steering rudder angles of ships are relatively small and hence the turning movements of the ship are small, the water current at the stern shows two steady patterns in some cases. This means that ship has two stable steady turning conditions for each rudder angle, and in the case of the ordinary automatic steering (also in the case of the manual steering) where the ship is moved straight ahead with a so-called small rudder angle making practically no turning movement, the movement of the ship does not take any definite pattern but results in a frequent unstable ship motion.